In recent years, because people have paid more attention to the energy problem, many new lighting tools with low energy consumption have been developed to save the energy. Therefore, the light emitting diode (LED) which has many advantages, such as high lighting efficiency, low power consumption, non-mercury and long lifetime, becomes the promising lighting tool of next generation.
The white LED used for lighting is produced by the blue LED chip and phosphor. The white light emitted from the white LED can be formed by using the blue light, which is emitted from the blue LED chip, to excite the yellow phosphor, for example, YAG (Yttrium Aluminum Garnet, Y3Al5O12), to emit the yellow light, and then mixing the two lights.
The phosphor coating methods comprise conformal coating method and remote phosphor coating method. The conformal coating method coats the phosphor on the surface of the LED chip to form the phosphor layer. Because the phosphor is directly coated on the LED chip, the phosphor layer has a uniform thickness. But, the light emitted from the phosphor layer may be absorbed by the LED chip and the carrier so the total lighting efficiency of the LED chip may be decreased. On the other hand, when the LED chip emits light, the temperature of the LED chip is around 100° C. to 150° C., and due to the direct contact of the LED chip and the phosphor layer, the heat produced by the LED chip may deteriorate the phosphor layer and decrease the transformation efficiency of the phosphor layer.
The remote phosphor method can solve the above mentioned problems of the conformal coating method. The remote phosphor method separates the phosphor layer and the LED chip to prevent the light emitted by the LED chip directly absorbed by the phosphor layer. And, due to the separation of the phosphor layer and the LED chip, the phosphor layer is not easy to be deteriorated by the heat produced by the LED chip.
When the particles of the phosphor layer absorb the light emitted by the LED chip, the particles are excited and emit the light with another color. However, the light emitted by the particles is omni-directional, including the light emitted toward the LED chip. Since the refractive index of the encapsulation resin is different from that of the phosphor layer, the total reflection of the light emitted by the particles happen easily so the light efficiency is decreased.